SPECT systems are often used to show the distribution of a radioactive substance inside a patient's body. A source of penetrating radiation is administered to the patient, which typically consists of a pharmaceutical tagged with a radionuclide which emits radiation photons (radiopharmaceutical). The radiopharmaceutical is designed to be absorbed in a target organ, such as the heart muscle, or other organs or body part of interest. The emitted radiation photons are collimated with a collimator subsystem and detected by a detector subsystem which generates output electrical signals which are digitized and processed by a computer system to generate images of the regional distribution of the radioactive sources in and around the target organ.
One prior SPECT system proposed by the inventor hereof utilizes a large circular shape design for the frame, or the gantry, collimator subsystem, and the detector subsystem which attempted to accommodate a large patient cross-section while placing the patient's heart at the geometric center for imaging from multiple directions simultaneously. However, because of the off-center location of heart, the circular geometry had to be fairly large to enclose a large patient's thorax. As a result, the long-distance collimation offsets the potential gain in geometric efficiency and renders the circular design less than optimum. Furthermore, the design devoted considerable collimator and detector area to the patient's right-posterior side, where the heart is too distant from the collimator for effective collimation. The typical problem of low photon sensitivity in SPECT is further compounded in cardiac imaging where the desirable radiation photons are scarce: only about 2-4% of the injected dose is absorbed in the myocardium of the heart. This circular design approach results in a limited return of the heavily attenuated and scattered photons and sub-optimal image quality.
Conventional and contemporary SPECT systems used and proposed for cardiac imaging suffer from a major weakness: these systems do not provide optimum detection coverage for photons emitted from the heart because they allow a large fraction of high quality photons to escape detection. This is well demonstrated by the requirement of using detector rotation around the patient, e.g., as utilized in conventional dual-head systems. Obviously, as detector rotates in incremental steps to catch photons on the far side of the patient, the photons on the near side escape coverage. Additionally, the detector area of prior systems has not been used efficiently for detection of photons emitted from the heart: most of the time, a large portion of the detector area is directed to the surrounding background area of the thorax.
Thus, an optimal system design for cardiac SPECT imaging needs to provide efficient and optimum detector coverage for high quality photons emitted from the heart, while effective collimation and adequate data sampling are achieved at the same time. Therefore, how to obtain an optimal balance between detector coverage, collimation, and sampling is the key to the design of a high-performance SPECT system.
The collimator subsystems of conventional SPECT systems are designed with only one predefined set of collimation parameters. For different imaging requirements, or for patient having different sizes, a different set of collimation parameters is often preferred. Therefore a different collimator with different collimation parameters is needed. However, changing the collimator during a conventional SPECT imaging procedure is not realistically feasible because such a procedure takes an inordinate amount of time and effort, and more importantly, it disturbs the patient's imaging position which is hard to be restored after the collimator change. The result is that conventional SPECT systems are not flexible in accommodating different collimation requirements to suit various clinical situations and patients having different sizes.
Additionally, conventional SPECT systems typically incrementally rotate the large, heavy collimator and the detector subsystem about the patient to obtain a plurality of projection images (projections). Each time the collimator and the detector subsystem are rotated step-by-step, the collimator and detector follow the patient's body contour by successively adjusting their radial and lateral positions. Such a technique is cumbersome, not easily reproducible, prone to both mechanical and electrical errors, slow, inefficient, utilizes expensive hardware to rotate large heavy collimator and detector subsystems, and requires extensive safety measures to protect the patient. As a result, conventional SPECT images have large variations in image quality and reproducibility, which make comparison of images from different facilities or from different times at the same facility difficult.